Conventionally, electronic displays that utilize a moving phenomenon of a colored liquid for display have been suggested. Examples of a system that utilizes an external electric field to move a liquid, thus displaying an image, include an electroosmosis system and an electrowetting system.
The electrowetting system utilizes a phenomenon in which an electric field is applied to a liquid inside capillaries so as to vary an interfacial tension of the liquid, thus moving the liquid along through holes by an electrocapillary phenomenon, whereas the electric field is removed, thus causing the liquid to flow out of the capillaries. In this system, in a voltage-ON state where a switch between an electrode provided on an inner surface of the capillary and an external electrode is closed, the electric field is applied to the liquid, so that a wettability of the liquid with respect to the inner surface of the capillary varies. Accordingly, a contact angle of the liquid with respect to the inner surface of the capillary decreases, and based on this phenomenon, the liquid moves inside the capillary by the electrocapillary phenomenon. On the other hand, when the switch is opened to remove the application of electric field to the liquid, the wettability of the liquid with respect to the inner surface of the capillary varies, thus increasing the contact angle sharply, and based on this phenomenon, the liquid flows out of the capillary.
In Patent document 1, which suggests an electrocapillary color display apparatus of this kind, colored electrically-conductive liquids that have different contact angles of droplets are used. Examples of them include water, alcohol, acetone, formamide, ethylene glycol and a mixture thereof.
Also, in Patent document 2 describing a display device and a display apparatus utilizing a similar electrowetting phenomenon, an aqueous solution in which an electrolyte such as NaCl or Na2SO4 is dissolved, a polar liquid such as water, alcohol, acetone, formaldehyde or ethylene glycol, or a mixture thereof with other suitable liquids are listed as the electrically-conductive liquid (an electrolyte solution). Also, in Patent document 3, water and an organic solvent are used as a dispersion medium of a coloring material whose base material is a pigment.
As described above, in the electrically-conductive colored liquids that are provided conventionally, water and an organic solvent often are used as the dispersion medium of the coloring material whose base material is a pigment. However, in the case where an aqueous dispersion medium is used, it is likely to permeate through a dielectric film covering the electrode surface because water itself has a small ionic radius, and a potential window is low, so that there is a problem that a dielectric breakdown occurs easily. In particular, in order to apply a voltage to the above-noted aqueous solution so as to vary its surface energy, it is necessary to perform high-voltage driving by applying a high voltage. Moreover, as the dielectric film becomes thinner, the dielectric breakdown is more likely to occur. Further, since the colored liquid itself has a vapor pressure, the vapor pressure of the liquid causes the aqueous dispersion medium to expand considerably in volume particularly in a high temperature region, leading to breakage. Furthermore, in the case where the dispersion medium contains combustibles, breakage due to an increase in internal pressure caused by shock or deterioration may lead to leakage or ignition of the electrolyte solution. Accordingly, handling is very difficult, and there is a problem in long-term durability.
Further, in the case of using water and an organic solvent as the dispersion medium of the coloring material whose base material is a pigment, temperature variations cause molecular motion of the dispersion medium itself to vary considerably, posing a problem that the agglomeration and precipitation of the coloring material occur.
Patent document 4 suggests using an ionic colored liquid having an anion and a cation instead of the above-described aqueous dispersion medium. That is, an ambient temperature molten salt is used as an electrolyte for producing a long-life highly-stable electrochromic device having polyaniline and polythiophene as the coloring material.
However, although Patent document 4 also has the problem of the agglomeration and precipitation of the coloring material, it fails to give any consideration to dispersing and stabilizing the coloring material in the ambient temperature molten salt.
Patent document 1: JP 10(1998)-39799 A
Patent document 2: JP 2000-356750 A
Patent document 3: JP 2003-221526 A
Patent document 4: JP 2004-527902 A